1. Field of Invention
This invention relates to AC to AC static power frequency changers such as cycloconverters and, more particularly, to apparatus for improving the quality of the output waveforms of such frequency changers through the use of feedback.
2. Prior Art
Static power frequency changers such as cycloconverters are electronic devices which synthesize an output waveform of a desired frequency and amplitude from a multiphase, essentially sinusoidal AC voltage of a different frequency by generating a plurality of component waveforms from selected portions of the individual phases of the source voltage. Static power switches connected in the several phases of the source voltage are rendered conductive sequentially by a pattern of signals developed by a control circuit at firing angles related to the phases of the source voltage in such a manner that the sum of the component waveforms produces an output waveform with a mean signal level that follows a reference voltage having the desired output frequency. The output waveform is applied to a filter, usually a series inductor with a shunt capacitor, which extracts the desired fundamental output frequency from the composite waveform.
When built with present day semiconductor components, a highly efficient (94% or better) frequency converter is realized. The waveform quality, high efficiency and light weight of the cycloconverter make it a suitable candidate for aircraft electrical power systems where it is used to convert the variable frequency of the engine driven generator to a fixed frequency. The filter in such a system is very important because it suppresses unwanted frequency components from the output and hence has a direct bearing on the waveform quality. However, the filter also respresents a significant portion of the system weight which imposes practical limits on the amount of filtering that can be provided.
As in any control system, the output, which in the case of the cycloconverter is the filtered output, will not faithfully represent the reference signal due to system errors. Some of the errors in the cycloconverter are introduced by the filter itself which would suggest using the filtered output as a feedback signal in a negative feedback loop to reduce the errors. However, control theory demonstrates that the phase shift through the filter could contribute up to 180.degree. to the closed loop phase lag and therefore create a severely unstable system. Even so, a small amount of feedback derived from the filtered output has been used successfully to provide some improvement in output waveform quality. In one such waveform improvement feedback scheme, the first derivative with respect to time of the output voltage as represented by the output filter capacitor current is used as the feedback signal. This approach effectively limits the filter's phase lag contribution to that of the filter's inductor only, or 90.degree.. Hence a 90.degree. improvement in system gain phase margin is realized permitting larger feedback gain and less error. This is a form of linear feedback that is commonly used in power conversion systems, however, it gives rise to a very nonlinear total loop gain, as will be discussed, which limits its effectiveness.
It is, therefore, a primary object of the present invention to improve the quality of the output waveform of a cycloconverter.
It is also an object of the invention to achieve improved cycloconverter output waveform quality through an improved form of feedback control.
It is another object of the invention to provide such feedback control which accommodates for the nonlinearity of the cycloconverter feed forward loop gain.
It is a more specific object of the invention to provide feedback which accommodates for the nonlinearity of the feed forward gain of the cycloconverter by varying the feedback gain as a function of the firing angle of the cycloconverter thyristor power circuit.
It is still another object of the invention to realize all of the foregoing objects through a scheme which is simple, reliable and inexpensive to implement.